An injection mold is provided with a gate and a cavity connected with the gate. A molten resin is injected into the cavity from the gate. When the injected molten resin has solidified in the cavity, the mold is opened to remove a molded product from the mold.
Resin shrinks when it solidifies. When the molten resin that has been filled in the cavity shrinks during solidification, the external shape of the molded product no longer matches the shape of the cavity. It is impossible to form the molded product into the desired external shape.
To solve this problem, the following technology was developed. This technology continues to apply pressure to the molten resin within the cavity from the gate while the molten resin within the cavity is solidifying. That is, the amount of molten resin necessary to make up for shrinkage is replenished. This technology can prevent the external surface of the molded product from separating from the cavity surface, even when the molten resin shrinks. As a result, a molded product having the desired external shape can be obtained.
A technology replacing the aforementioned technology is disclosed in Japanese patent application publication No. H10-58493. This technology focuses on the fact that many molded products have both a surface that must be finished to a desired shape (this surface is called a “design surface”), and a surface whose finish is not important (this surface is called a “back surface”). In this prior art method, when the filling of the cavity with the molten resin from the gate is finished, the pressure being applied to the molten resin is stopped. A pressurized fluid is injected toward the back surface of the molded product. When the pressurized fluid is injected toward the back surface of the molded product, the back surface separates from the cavity surface, but the design surface of the molded product is pushed against the cavity surface. Consequently, the design surface of the molded product can be finished to the desired shape. It is explained that this technology does not require an extra amount of resin, since the pressure being applied to the molten resin from the gate is stopped when the cavity has been filled with the molten resin.